Method and device of electrical power

ABSTRACT

A power factor correction of three-phase boost-type conversion is disclosed. Embodiments comprising multi-leg autotransformers are disclosed, e.g. comprising 3-phase low-pass filtering impedances such as capacitors between an input of a converter and a midpoint of the output.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2010/075564, filed on Jul. 29, 2010, which claims priority toInternational Application No. PCT/SE2010/000140, filed on May 24, 2010,both of which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to power converter and multi-phaseboost-type converters for power factor correction.

BACKGROUND

Power supplies for telecommunication equipment, are required to delivera DC voltage for any type of application. The power supplies are poweredby the AC-mains, where the voltage is rectified by means of a bridgerectifier, (other types) etc.

Power factor correction, PFC, circuits are often used in switched modepower supplies and rectifiers connected to the AC-mains. The PFC circuitreduces the harmonic contents in the current drawn from the mains, andcontrols the power factor to be close to unity. For this purpose, aboost converter is often used; especially at higher power levels.

Y. Zhao, Y. Li, and T. A. Lipo, “Force commutated three level boost typerectifier,” IEEE Industry Applications Society Annual Meeting, pp.771-777, vol. 2, 199; and J. W. Kolar e H. Ertl, “Status of thetechniques of three-phase rectifier systems with low effects on themains,” IEEE International Telecommunications Energy Conference, p. 16,1999 describes a three-level boost converter utility interface formingthe basis of most three-phase boost converters found in the markettoday. The utility interface draws nearly sinusoidal current from athree phase utility with a power factor near unity.

U.S. Pat. No. 4,268,899, discloses single phase and three phaseembodiments of a rectifier circuit operable as a full wave bridge or asa phase controlled voltage doubler depending on the line-loadconditions. The circuit has its greatest power factor at high lineconditions.

When high power is being processed, semiconductors like IGBT, MOSFET,diodes, GTO, MCT and others have been the chosen solution for the activeswitches in the applications found in the industry. However, using thosedevices has been related with many issues that are limiting theefficiency and/or power density like:

-   -   a. Current sharing between paralleled devices    -   b. Reduction of the switching frequency due to the increased        commutation losses which increases the weight and the size of        the unit.

Another drawback of the existing solutions is that the EMI-levels aretoo high and requires several stages in the input filter in order toreduce both the CM- and DM-noise. This reduces the performance andincreases the volume/cost of the unit.

G. V. T. Bascopé e Ivo Barbi, “Generation of a family of non-isolatedDC-DC PWM converters using a three-state switching cell”, IEEE 31thAnnual Power Electronics Specialists Conference, Volume: 2, pp. 858-863,18-23 Jun., 2000, incorporated herein by reference, describes theconcept of the three-state switching cell, 3SSC.

G. V. Torrico-Bascopé e I. Barbi, “A single phase PFC 3 kW converterusing a three-state switching cell”, IEEE 35th Annual Power ElectronicsSpecialists Conference, Volume: 5, pp. 4037-4042, 20-25 Jun., 2004,describes an application of the 3SSC in a single-phase PFC circuit.

SUMMARY

One embodiment discloses a method of power factor correction of a3-phase power converter. The method includes connecting each phase to amidpoint across an autotransformer.

A second embodiment discloses a device including an autotransformerinter-connecting each phase to a midpoint. The device corrects powerfactor of a 3-phase power converter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of an embodiment of a three-phaseboost converter constructed in accordance with the disclosure.

FIG. 2 illustrates example basic formats of bi-directional switches ofFIG. 1 in accordance with the disclosure.

FIG. 3 illustrates one cycle of three phase voltages and an indicatedpoint in time for example analysis/illustration of modes of operation ofan embodiment in accordance with the disclosure.

FIG. 4 illustrates a first mode of operation according to a firstsetting of bidirectional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1)and S_(c2) of an embodiment of the disclosure.

FIG. 5 illustrates a second mode of operation according to a secondsetting of bidirectional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1)and S_(c2) of an embodiment of the disclosure.

FIG. 6 illustrates a third mode of operation according to a thirdsetting of bidirectional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1)and S_(c2) of an embodiment of the disclosure.

FIG. 7 illustrates a fourth mode of operation according to a fourthsetting of bidirectional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1)and S_(c2) of an embodiment of the disclosure.

FIG. 8 illustrates a fifth mode of operation according to a fifthsetting of bidirectional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1)and S_(c2) of an embodiment of the disclosure.

FIG. 9 illustrates a sixth mode of operation according to a sixthsetting of bidirectional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1)and S_(c2) of an embodiment of the disclosure.

FIG. 10 illustrates a seventh mode of operation according to a seventhsetting of bidirectional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1)and S_(c2) of an embodiment of the disclosure.

DETAILED DESCRIPTION

The developing trend of the front-end three-phase converters is highefficiency and high power density. However, achieving high efficiencyoften results in lower power density.

Paralleling of converters has been a way to solve the demand ofincreasing DC-power, but due to the issues mentioned in the prior-artsection, this is not an optimal solution for achieving high efficiencyand high density.

Increasing the number of semiconductors, magnetic components and amountof copper; it would be possible as such to achieve high efficiency withprior-art circuit topologies on which circuits existing in the markettoday are based. This, however, would decrease power density. If aprior-art converter is to be optimized for high density, the efficiencywill definitely be lower.

As a consequence, the choice for optimization in the existing solutionsin today's prior art is either for high efficiency or high density, butnot both.

Consequently, it is an object of embodied examples in accordance withthe disclosure to provide circuits capable of achieving both highefficiency and high density.

A further object of an embodied multilevel circuit configuration is tofacilitate reduced voltage and the current stress on the semiconductors.

Also, an object of embodied multilevel circuit configuration is tofacilitate size, weight, and/or volume reduction of a converter, heatsink, input filter and/or reactive components of the converter unit.

Finally, it is an object of embodiments in accordance with thedisclosure to facilitate use of semiconductors of relaxed requirementsas compared to prior-art circuit topologies, given power-converterspecifications.

A new topological circuit is presented, embodiments of which are capableof overcoming this inherent characteristic of prior art front-endthree-phase converters. Example embodiments of this new converterdemonstrate very high efficiency and high power density, as furtherexplained in the detailed description.

Multi-level converters, lower the maximum voltage over the activeswitches with the number of voltage levels.

The disclosed multi-level topology is not limited to a particular numberof levels. A three-level boost converter, however, provides a practicaltopology for use in high power, high input voltage, front end PFCcircuits and facilitates fulfillment of the requirement of highefficiency. The main drawback of prior-art topologies based on thethree-level boost converter, is that the EMI (ElectromagneticInterference) levels are too high. Embodiments of the disclosure reducethis drawback.

Embodied example circuits disclosed in this specification are capable ofachieving both high efficiency and high density.

An approach that is most suitable for high power is using converterswith multilevel features for voltage and current instead.

An advantage of an embodied multilevel circuit configuration is that thevoltage and the current stress on the semiconductors are reduced withthe increased number of levels in the converter. This will reduce therequired cooling and thereby the size and volume of the unit.

Also, size, weight and volume of the reactive components of exampleembodiments are substantially reduced compared to what is the case inprior-art technology due to the reactive components operating at higherperceived frequency.

The limitations as described above and found in front-end convertersused in the industry today have been solved by an embodied multilevelconverter of circuit topology in accordance with the disclosure.

At the same time, the embodied multilevel converter improves theEMI-performance, compared to what is achieved according to prior-arttopologies. This facilitates, e.g., reduction of weight, size and volumeof an input filter and thereby of a converter unit.

An object of an embodiment of the disclosure to provide a noveltopological circuit for a Three-Phase Boost Converter to use indifferent applications where high efficiency and high density arerequired. It comprises multi-state (e.g. five-state) switching cell.

This novel topological circuit is based on the 3SSC (Three-StateSwitching Cell). As described above, the main characteristic of thiscircuit is the possibility to achieve high efficiency and high powerdensity, due to the fact that the reactive components work with twicethe switching frequency.

Due to the inherent characteristic of the 3SSC, the peak currentsthrough the semiconductors are lower compared to prior-art circuittopologies and consequently the conduction and switching losses arelowered. This allows us to use cost effective semiconductors and smallerheat-sinks which will also reduce the total size of the unit.

Referring initially to FIG. 1, illustrated is a schematic diagram of anembodiment of a three-phase boost converter constructed in accordancewith the disclosure. The boost converter receives input power from athree-phase voltage source (1) (via an EMI-filter, (2)). The examplecapacitors C_(c1)-C_(c3) (3) are connected in a wye, or Y, configurationbetween the EMI-filter and the boost inductors, L_(a), L_(b), L_(c) tothe midpoint, M, (8) of the converter. It should be noted that havingcapacitor in a Y-configuration is a preferred mode. This does notexclude more general impedances or low-pass filters, nor does it requirea particular Y configuration as long as the mid-point could be fixed toa corresponding reference through circuitry providing low-pass filtercharacteristics of the voltage. In an example realization, smallcapacitors in the range of some 300 nF or less, e.g. 220 nF, providegood performance in relation to other parameters of relevance. Thislow-pass filtering will reduce the “noise” of sample values of asinusoid input voltage due to switching. It is substantially differentfrom the EMC (Electromagnetic Compatibility) filtering providingfiltering of the parasitic capacitance that commonly exists betweenactive devices and a heat sink. The input power goes through first,second and third boost inductors L_(a), L_(b) and L_(c) (4) and theautotransformers T_(a), T_(b) and T_(c) (5) that are coupled to arectifier (6) which includes a plurality of diodes, D_(a1)-D_(a4),D_(b1)-D_(b4) and D_(c1)-D_(c4), arranged in a full bridgeconfiguration. In the example illustration the autotransformers comprisetwo transformer windings or legs, each leg connected to the midpointacross a bi-directional switch (7). The rectifier (6) rectifies thethree-phase input voltage into a DC voltage for application to thebi-directional controllable switches (7), S_(a1), S_(a2), S_(b1),S_(b2), S_(c1) and S_(c2) connected to the midpoint, M (8).

A (optional) boost inductor, as L_(a) (4), an autotransformer, as T_(a)(5), rectifier diodes, as D_(a1)-D_(a4) (6) and bi-directional switchesas S_(a1), S_(a2) (7) are building a multi-state switching cell asillustrated and “highlighted” within dashes in FIG. 1.

For each leg, the example configuration of FIG. 1 provides two diodes.The disclosure covers topologies comprising three or more legs, evenautotransformers comprising two-legs are illustrated for reasons ofsimplicity. Having, e.g., three legs the “highlight” area would comprise6 diodes (6) and 6 bi-directional switches (7). With, e.g., four or fivelegs the figure would translate to 8 or 10 diodes (6), respectively, andfour or five bi-directional switches (7), respectively, etc. The greaterthe number of legs, the closer to samples of a sinusoid the switchedvoltage samples will be. Also, the more legs there are the greater isthe effective frequency seen by components such as the boost inductors(4). Since the impedance of the inductors depends on frequency on thesame scale as inductance, this may be reduced in relation to the numberof legs. Of course, the relative reduction for each additional leg willreduce with number of legs included.

The DC-output voltage, referred to as three level output voltage, ischarging two groups of capacitors, C₁ and C₂ (8) connected in seriesbetween P and M and N and M respectively. The capacitors are preferablyset depending on the load to fulfill required hold-up time and may foran example realization be set to less than 300 μF, e.g. 270 μF.

The bi-directional switches S_(a1), S_(a2), S_(b1), S_(b2), S_(c1) andS_(c2) (7) can have for instance the basic formats as schematicallyillustrated in FIG. 2. To be bi-directional, the switches each comprisetwo transistors. The diodes of example realization a) are inherent,whereas the diodes of example realizations b)-d) are not. Consequently,the actual realization of a) may be seen as a′). This realization isalso the preferred one out of the examples illustrated as it in generalprovides the smallest resistive loss. The bi-directional switch isopened or closed by the (square) control port.

The embodied Converter is controlled by Continuous Conduction Mode ofOperation, CCM. For the control of the converter, Space VectorModulation or Carrier based Control may be implemented.

The operation of the converter can be divided into 7 different modes ofoperation during one half of the total switching period. Example sevenmodes are provided for an example point in time of the cycle asillustrated in FIG. 3. As the function of the converter in the otherhalf of the total switching period is symmetrical, only 7 modes areillustrated as in FIG. 4 to FIG. 10. In the figures, the switches arepreferably realized according to example a′) of FIG. 2, and switched bythe control current. To simplify reading, leading diodes have beenfilled and non-leading paths have been dashed.

In this description, certain acronyms and concepts widely adopted withinthe technical field have been applied in order to facilitateunderstanding. The disclosure is not limited to units or devices due tobeing provided particular names or labels. It applies to all methods anddevices operating correspondingly. This also holds in relation to thevarious systems that the acronyms might be associated with.

While the disclosure has been described in connection with specificembodiments thereof, it will be understood that it is capable ofcombining the various embodiments, or features thereof, as well as offurther modifications. This specification is intended to cover anyvariations, uses, adaptations or implementations of the disclosure; notexcluding software enabled units and devices, processing in differentsequential order where non-critical, or mutually non-exclusivecombinations of features or embodiments; within the scope of subsequentclaims following, in general, the principles of the disclosure as wouldbe obvious to a person skilled in the art to which the disclosurepertains.

1. A 3-phase power converter comprising: a three-phase voltage source,an EMI-filter, capacitors C_(c1)-C_(c3), boost inductors L_(a), L_(b)and L_(c), autotransformers T_(a), T_(b) and T_(c), a rectifier andbi-directional controllable switches, wherein the 3-phase powerconverter receives input power from the three-phase voltage source viathe EMI-filter, the capacitors C_(c1)-C_(c3) are connected between theEMI-filter and the boost inductors, L_(a), L_(b), L_(c) to a midpoint ofthe 3-phase power converter, the input power goes through first, secondand third boost inductors L_(a), L_(b) and L_(c) and theautotransformers T_(a), T_(b) and T_(c) that are coupled to a rectifier,and the rectifier rectifies the three-phase input voltage into a DCvoltage for application to the bi-directional controllable switchesconnected to the midpoint of the 3-phase power converter.
 2. The 3-phasepower converter according to claim 1, wherein the three capacitorsC_(c1)-C_(c3) are connected in a wye, or Y, configuration between theEMI-filter and the boost inductors, L_(a), L_(b), L_(c) to the midpointof the 3-phase power converter.
 3. The 3-phase power converter accordingto claim 1, wherein the rectifier comprises a plurality of diodes,D_(a1)-D_(a4), D_(b1)-D_(b4) and D_(c1)-D_(c4), arranged in a fullbridge configuration.
 4. The 3-phase power converter according to claim3, wherein the bi-directional controllable switches, each connected to aleg of the autotransformer, the autotransformer and diodes comprised inthe rectifier form a multi-state switching cell.